Communication networks transfer information, such as data, voice, text or video information, among communication devices connected to the networks. Most enterprises employ local area networks (LANs), such as those based on the Ethernet protocol, to interconnect the various devices within the enterprise. Most LANs are connectionless, where frames are transmitted without error to a high degree of probability, but there is no guarantee of delivery. If a frame is not properly received, the receiving station will simply discard the frame without notifying the sender.
In addition, even when a frame is successfully transmitted, a frame can be lost due to buffer unavailability at the receiving station. If a receiving station is unable to receive and process data at a rate greater than or equal to the transmission rate of the transmitting station, the buffer at the receiving station can overflow. The loss of a frame due to buffer unavailability has the same effect as a frame that is lost due to a bit error. Thus, it is desirable to suspend or reduce the arrival of data packets rather than permit an overflow condition to occur at the buffer. A number of techniques have been proposed or suggested for flow control in LANs.
The IEEE 802.3x data communication protocol specifies a port-based flow control arrangement for full duplex Ethernet links based on a flow control message, such as a “pause” frame. A transmitting station that receives the pause frame enters a pause state in which the transmission of frames on the network is suspended for a specified time, thereby relieving congestion at the receiver. For a more detailed discussion of the pause function, see, for example, R. Seifert, Gigabit Ethernet: Technology and Applications for High-Speed LANs, 100-11 (1998), incorporated by reference herein. The pause function was contemplated for use by two end stations communicating over a dedicated full duplex Ethernet link. A pause frame includes a destination address in the form of a predefined multicast address. The use of a multicast address relieves the sender of a pause frame from having to know the specific address of the other device on the link.
A number of LANs are often connected in a larger network, such as a wide area network (WAN). While the Ethernet protocol is well suited to the LAN environment, the Ethernet protocol is not a viable option for WANs, primarily because the Ethernet collision avoidance mechanism limits the permissible distance of the furthest station and privacy issues related to separation of traffic over a transport network. Thus, in order to permit communication between LANs in an efficient, seamless and reliable manner, high-speed transport links between various LANs was needed.
In order to meet increased bandwidth demands, fiber optic systems, such as Synchronized Optical Networks (SONETs), are often employed as transport links in a WAN. Typically, a number of Ethernet packets from one or more source LANs are aggregated for transport over a SONET link to one or more destination LANs. If a station that is receiving packets from a remote transmitting station over a WAN link is unable to receive and process the packets at a rate greater than or equal to the transmission rate of the remote transmitting station, the buffer at the receiving station can overflow.
A port-based pause function based on the IEEE 802.3x data communication protocol has been proposed to control the flow of packets aggregated over a transport link. The multicast address of a pause frame, however, would cause the pause frame (if delivered at all) to be broadcast to all transmitting stations associated with the aggregated packets of a given flow. Thus, all such transmitting stations that receive the pause frame will suspend the transmission of frames even though only the buffer of one receiving station has a potential overflow condition. Thus, a need exists for an improved method for controlling the flow of packets aggregated from multiple logical ports over a transport link. A further need exists for a method for suspending the flow of only a single station that transmits packets over an aggregated transport link.